1. Field of the Invention
The present invention relates to a method for forming an image by an electrophotographic method or an electrostatic recording method using a toner for developing an electrostatic image. More specifically, the invention relates to a method for forming an image suitable for graphic arts and short run printing.
2. Description of the Related Art
Such a method of visualizing image information through an electrostatic latent image as an electrophotographic method has been utilized in various fields including duplicators and printers associated with the progress of the technique thereof and the growth of the market.
In the electrophotographic method, an electrostatic latent image is formed on a photoreceptor through a charging step and an exposing step, and the electrostatic latent image is developed with a developer containing a toner and then visualized through a transferring step and a fixing step.
The developer used herein includes a two-component developer containing a toner and a carrier, and a one-component developer using a magnetic toner or a non-magnetic toner solely.
The toner is generally produced by a kneading and pulverizing method, in which a thermoplastic resin is mixed and kneaded with a pigment, a charge controlling agent, a releasing agent and the like, and after cooling the resulting mixture, it is pulverized and classified. Inorganic or organic particles may be added to the surface of the toner for improving the flowability and the cleaning property thereof depending on necessity. A toner produced by the ordinary kneading and pulverizing method has an irregular shape and an irregular surface structure, which are delicately changed depending on the pulverization property of the material used and the pulverization conditions, and therefore, the shape and the surface structure of the toner are difficult to be controlled intentionally. In the case where a material having high pulverization property is used, it often causes formation of further powder and changes in toner shape due to a mechanical force in a developing device. Due to the influences of these phenomena, in the two-component developer, the powder is fused to the surface of the carrier to accelerate charge deterioration of the developer. In the one-component developer, the particle size distribution is broadened to cause scattering of the toner, and the toner shape is changed to lower the developing property to cause deterioration in image quality.
In the case where a toner is internally added with a releasing agent, such as wax, the releasing agent is often exposed to the surface of the toner depending on the combination with the thermoplastic resin to exhibit influence. Particularly, in a combination of a resin that is slightly difficult to be pulverized by imparting elasticity with a high molecular weight component and a brittle wax releasing agent, such as polyethylene, polyethylene is often exposed to the surface of the toner. The phenomenon is advantageous in releasing property upon fixing and cleaning property of a non-transferred toner from a photoreceptor, but the polyethylene on the surface easily migrates by a mechanical force to contaminate a developer roll, a photoreceptor and a carrier, so as to bring about deterioration in reliability. Furthermore, the toner is insufficient in flowability due to the irregular shape thereof even by adding a flowability assistant, whereby particles on the surface of the toner migrate to recessed parts on the surface by a mechanical force during use to lower the flowability with the lapse of time, and the flowability assistant is buried in the toner to deteriorate the developing property, the transferring property and the cleaning property. Moreover, in the case where a toner recovered upon cleaning is returned and reused in the developing machine, deterioration in image quality is liable to occur. When the amount of the flowability assistant is increased to prevent these problems, other problems occur that black spots are formed on the photoreceptor, and the assistant particles are scattered.
In recent years, production processes of toners by an emulsion polymerization aggregation method have been proposed as a method for intentionally controlling the shape and the surface structure of the toner, as decribed in JP-A-63-282752 and JP-A-6-250439.
In the emulsion polymerization aggregation method, materials having been formed into particles having a diameter of 1 μm or less are generally used as starting materials, and therefore, a toner having a diameter of about from 1 to 25 μm can be theoretically produced. More specifically, in general, a resin dispersion liquid is formed by emulsion polymerization, and a colorant dispersion liquid having a colorant dispersed in a solvent is separately produced. The resin dispersion liquid and the colorant dispersion liquid are mixed to form aggregated particles having a diameter corresponding to a toner diameter, and thereafter, the aggregated particles are fused and integrated by heating to produce a toner. In this method, the toner surface has the same composition as the interior thereof, and it is difficult to control the surface composition intentionally.
In order to solve the problem, it has been proposed that the compositions over the inner layer to the surface layer are freely controlled even in the toner produced by the emulsion polymerization aggregation method to realize more accurate control of the particle structure as described in JP-A-10-26842. Furthermore, the inventors have proposed crystalline toners using a crystalline resin as a binder resin in JP-A-2001-117268 and JP-A-2003-98736. These toners can be easily reduced in diameter and realize accurate control of the particle structure, whereby the image quality is significantly improved in comparison to the conventional electrophotographic images, and simultaneously high reliability is expected.
In recent years, the image forming method by electrophotography using the aforementioned toner and developer techniques is being applied to a part of the field of printing associated with digitalization and colorization thereof, and is being pronouncedly practiced in the fields of graphic arts and short run printing including on-demand printing.
The term “filed of graphic arts” referred herein means fields of business relating to mass production of printed matters, which is referred to as reproduction and duplication, of creative printing with a small number of copies, such as engrave printing, and the original art works, such as calligraphy and painting, among the ordinary fields of business and sectors relating to production of printed matters.
In the field of short run printing, in addition to the monochrome printing, such a technology has been developed that targets short run color printing represented by Color Docu Tech 60 by Fuji Xerox Co., Ltd. by exploiting the characteristic features of the electrophotographic method, and major breakthroughs have occurred in image quality, dealing with transfer paper, product price, and cost per one printed sheet (as described in Journal of the Imaging Society of Japan, vol. 40, No. 2 (2001)).
In comparison to the conventional full-fledged printing, however, the electrophotographic image forming method is liable to involve problems, although the on-demand characteristics are exploited owing to printing using no printing plate, in the image quality including color reproduction range, resolution and glossiness, the texture, the uniformity within one image, the maintenance of image quality upon continuous printing for a long period of time, the high cost per one sheet due to the large toner consumption amount for high image density, the dealing with thinner or thicker paper, the image defect and poor writing capability due to an oil remaining after fixation of an image, and elongation, curling and wrinkle of transfer paper and deviation in registers upon double face printing due to image fixation at high temperature and high pressure. Furthermore, because a toner image formed of a low molecular weight resin having a relatively low melting point is fixed with heat as the principal of the method, there are some cases where the resulting image has poor resistance to heat and mechanical stress in comparison to the ordinarily printed image. Therefore, in the case where the image is exposed under high pressure and high temperature after folded several times over or bound as a book, there are some cases where problems occurs in durability against various kinds of stress, such as defects in image, blocking, offset, deteriorated light resistance upon outdoor exposure and deteriorated weather resistance.
As having been described, it has been found that there are many problems in the case where the electrophotographic image formation is utilized as a commercially valuable productive property in the field of graphic arts to substitute the ordinary full-fledged printing.
In order to improve the color reproduction range, colorants having higher performance are necessary since the pigments practically used in the field of electrophotography are poor in variation in comparison to those used in the conventional printing ink. There are a wide range of service conditions in the field of graphic arts as compared to the office market, and therefore images necessarily have various kinds of durability including heat resistance, light resistance, oil resistance, solvent resistance, scratch resistance folding resistance, in addition to high color reproducibility.
In the image processing system and the system including a photoreceptor and exposure thereof, the resolution of the image is liable to be influenced by the particle diameter of the toner and the particle diameter distribution thereof, and there are major technical problems in the case where a toner having a smaller diameter is used effectively with high reliability in respective process steps of charging, developing, transferring, fixing and cleaning.
For example, there are rooms to improve in such systems for dealing with the toner having a small diameter as design of a carrier, a charging blade or a charging roll for uniformly charging the toner having a small diameter, a developing system for obtaining a high density image without occurrence of background stain, a transferring system for realizing fine transferring with high efficiency, a fixing system for dealing with combinations of various kinds of paper with the toner having a small diameter, and a cleaning system for removing the toner having a smaller diameter completely from the photoreceptor or the intermediate transfer material to realize stable image quality.
In order to improve the in-plane uniformity and defects of an image, it is important to control the uniformity in developing capability of a developer in the developing system. In order to attain the commercial demand in maintenance of image quality, such a developer is necessary that maintains stable charging property and stable and uniform development during continuous printing exceeding several thousands of sheets, and that has high durability with less dependency to the environment including temperature and humidity, and the developing system with high durability is necessarily optimized for maintaining the uniform in-plane density by avoiding influences of paper powder and foreign matters and preventing defects and noises from occurring.
As the transferring system for transferring an image from the photoreceptor or the intermediate transfer material, an electrostatic transferring system is ordinarily used in the current electrophotographic system. In the case of a color image having a large thickness due to accumulation of toners of respective colors, it is necessary that the precise control of the behavior of the toner in an electric field is optimized in order to suppress deterioration of the image due to scattering of the toner upon transferring, and in some cases, such a transferring system is necessary that can drastically prevent scattering of the toner by using adhesion transferring without an electrostatic force.
It is also necessary that the cleaning system is optimized from the toner material, the structure and the hardware system in order to clean continuously the toner having a small diameter with a controlled shape, such as a spherical shape, with high reliability without environmental dependency by blade cleaning, electrostatic brush cleaning, magnetic brush cleaning, web cleaning and cleaning simultaneously with development, associated with a photoreceptor having high durability.
In response to the demand for decreasing the cost per one sheet, it is necessary to decrease the toner consumption amount by decreasing the toner diameter and optimizing the amount of the colorant, which are liable to influence on the uniformity in image quality. It is important for decreasing the cost per one sheet that the amount of disposal output for obtaining stable image quality, which largely influences on the cost upon printing, is decreased, and the maintenance load is decreased, by realizing the aforementioned highly reliable image formation system.
In order to deal with thinner or thicker paper, it is necessary that paper having poor stiffness, such as thin paper, or a plastic film can be easily released from the fixing member, such as a fixing roll, after fixing, and such a toner is used that is capable of being fixed at a low temperature to suppress the electric power consumption upon fixing coated paper or thicker paper. The stress on the transfer paper can be reduced by lowering the temperature and the pressure upon fixing, and thus elongation, curling and wrinkle of transfer paper and deviation in registers can be resolved. In order to avoid image defects, such as spots and stripes, and poor writing capability, due to an oil, it is necessary to use an oilless fixing device or an oilless toner containing a releasing agent inside the toner.
Furthermore, in order to attain image durability causing no problem under various use conditions as being equivalent to the ordinary printed image, it is necessary that the characteristics of the resin used in the conventional toner are largely improved.
In order to improve the degree of freedom and the uniformity of the glossiness characteristics, it is necessary that the fixing device is optimized associated with control of the viscoelasticity of the toner. In order to obtain high grade image quality equivalent to the offset printing, it is important that such glossiness is realized that is optimized to paper used to improve the market value, and thus, the combination of the toner, the paper and the fixing system is necessarily optimized.
In the filed of on-demand printing, the environmental load capability thereof is demanded to be improved in recent years. In the case where the printing is carried out by an on-demand operation, the stocks can be omitted or minimized to lower the environmental load associated with stocks, transportation and disposal of printed matters, which are liable to occur in the ordinary printing operation. The dry toner used in the ordinary electrophotography does not contain an organic solvent, which is used in an ink for the ordinary printing machine, whereby the environmental load associated with VOC can be drastically reduced. In order to attain further improvement, however, it is important that not only the electric energy used on fixing an image and on maintaining the hardware conditions is reduced, but also odors and volatile matters generated from the resin fused by heating upon fixing and substances with questions arising in carcinogenicity and environmental endocrine disruption are lowered or avoided, and the toner components having smaller diameters are prevented from being exhausted to the exterior of the machine. Furthermore, the recyclability of the waste toner and printing paper are also necessarily considered.
As having been described, in order to deal with the demands in the fields of graphic arts and short run printing, it is necessary that the conventional electrophotography techniques are further advanced as a total system.
In the image forming method by electrophotography, the image is generally fixed by heating to a temperature far exceeding 100° C., which is the boiling point of water. Under the aforementioned demands, such a phenomenon brings about a fatal problem in the field of graphic arts and short run printing that fixation unevenness and roughness in images caused by blister (roughness on image surface due to vaporization of moisture) occur due to vaporization of moisture from the transfer paper upon fixing, so as to provide unevenness in images and color fluctuation within the same page or among pages, which deteriorate the grade of the image.